This invention relates to oil flow sensing.
In a two-cycle outboard marine engine, for example, lubricating oil flows in pulses, rather than continuously. The oil may be pumped by a solenoid that is triggered by an electrical signal from an engine control module (ECM). The pulses occur at a rate that depends on engine speed and may be as high as 4 Hz or higher or as low as 0.007 Hz with a pulse duration of about 40 milliseconds.
One way to make sure that oil is always reaching the parts of the engine that need lubrication is to include level switches in an oil reservoir. When the level of the oil falls unacceptably low, an alarm can be triggered or the engine can be stopped.
In general, in one aspect, the invention features a method that includes (a) passing an electrical current through a thermistor to raise its temperature above the temperature of oil flowing in pulses past the thermistor, (b) measuring a change in temperature of the thermistor occurring with respect to one or more of the pulses, (c) determining a level of oil flow corresponding to the measured change in temperature, and (d) issuing a signal based on the determined flow level.
Implementations of the invention may include one or more of the following features. Measuring the change in temperature includes measuring a change in voltage across the thermistor over a period of time. The period of time corresponds to different portions of at least one of the pulses. The period of time begins at the start of one of the pulses and ends no later than the start of the next one of the pulses. The thermistor is housed in a package having an area that yields an oil flow of 10 to 20 inches per second. The area is in the range of 0.0005 to 0.002 square inches. The oil is flowing in a 2-cycle marine engine. A signal indicative of the timing of the pulses is received from an electronic control module of the engine. The signal based on the determined flow level is sent to an electronic control module of the engine. The rate of pulses is as high as 5 Hz The rate of pulses is as low as 1 pulse per day.
In general, in another aspect, the invention features an apparatus that includes a coupling having (a) two open ends adapted for connection to upstream and downstream tubes of a pulsating oil circulation system of an engine and (b) a channel configured to direct the oil to flow past a thermistor connected to a sensing circuit. The sensing circuit includes elements connected to determine a change in a voltage across the thermistor and to compare the change to a threshold.
Implementations of the invention may include one or more of the following features. The sensing circuit includes a sample-and-hold circuit connected to store a voltage across the thermistor. The sensing circuit includes a delay circuit connected to provide timing signals for the period over which the change in voltage is determined. In some implementations, the sensing circuit is a microcontroller that includes an analog-to-digital converter. Ports are connected to carry timing and flow-state signals between the sensing circuit and a control circuit of the engine.
In general, in another aspect, the invention features a marine engine that includes (a) moving parts arranged to be lubricated by oil delivered through a supply line from a supply of oil, (b) a pump configured to pump oil from the supply to the moving parts in pulses controlled by a controller, and (c) a sensor connected to receive pulses of the oil and to detect the oil flow state of the engine using a temperature sensitive electronic element and a circuit that analyzes an electrical parameter of the temperature sensitive element at times based on the pulses of the oil.
Implementations of the invention may include one or more of the following features. The temperature sensitive electronic element includes a thermistor. The circuit includes an electrical interface to a controller that controls the timing of the pulses.
Among the advantages of implementations of the invention, the response time of the sensing circuit is short, only a single thermistor is required (because the flow is pulsating), there are no moving parts, and the device is insensitive to mounting orientation and vibration.
Other advantages and features will become apparent from the following description and from the claims.